CO2 insertion into metal-carbon bonds: a computational study of Rh(I) pincer complexes

Catalytic carboxylation reactions that use CO(2) as a C1 building block are still among the 'dream reactions' of molecular catalysis. To obtain a deeper insight into the factors that control the fundamental steps of potential catalytic cycles, we performed a detailed computational study of the insertion reaction of CO(2) into rhodium-alkyl bonds. The minima and transition-state geometries for 38 pincer-type complexes were characterized and the according energies for the C-C bond-forming step were determined. The electronic properties of the Rh-alkyl bond were found to be more important for the magnitude of the activation barrier than the interaction between rhodium and CO(2). The charge of the alkyl-chain carbon atom, as well as agostic and orbital interactions with the rhodium, exhibit the most pronounced influence on the energy of the transition states for the CO(2) insertion reaction. By varying the backbone and the donor groups of the pincer ligand those properties can be tuned over a very broad range. Thus, it is possible to match the electronic and steric properties with the fundamental requirements of the CO(2) insertion into rhodium-alkyl bonds of the ligand framework.